Basis for original (SCP) IDLH: The available toxicological data show no evidence that indicates an acute exposure to a high concentration of crystalline silica would impede escape or cause any irreversible health effects within 30 minutes. For this draft technical standard, therefore, respirators have been selected on the basis of the assigned protection factor afforded by each device. However, for some particulate substances for which no evidence of an IDLH exists, the determination of allowable respiratory protection based on protection factors may result in the assignment of respirators for concentrations that are not likely to be encountered in the occupational environment. Therefore, for all such particulate substances it has been arbitrarily determined that only the “most protective” respirators are permitted for use in concentrations exceeding 500 ´ the OSHA PEL.

Short-term exposure guidelines: None developed

ACUTE TOXICITY DATA:

Animal or human data: None relevant for use in determining the revised IDLH.

Revised IDLH: 25 mg/m3 (Cristobalite & Tridymite); 50 mg/m3 (Quartz & Tripoli)Basis for revised IDLH: The available toxicological data contain no evidence that an acute exposure to a high concentration of crystalline silica would impede escape or cause any irreversible health effects within 30 minutes. However, the revised IDLHs for crystalline silica are 25 mg/m3 for Cristobalite and Tridymite and 50 mg/m3 for Quartz and Tripoli, based on being 500 times the 1989 OSHA PELs of 0.05 mg/m3 and 0.1 mg/m3, respectively (500 is an assigned protection factor for respirators and was used arbitrarily during the Standards Completion Program for deciding when the “most protective” respirators should be used for particulates). [Note: NIOSH recommends that the “most protective” respirators be worn for all varieties of crystalline silica at concentrations above 25 mg/m3.]